System and methods for high-rate co-sputtering of thin film layers on photovoltaic module substrates

ABSTRACT

Systems and methods for deposition of a thin film layer on photovoltaic (PV) module substrates are generally provided. The system can include a sputtering chamber configured to receive the substrates, at least two targets positioned within the sputtering chamber, and an independent power source connected to each target. Each target can be positioned within the sputtering chamber to face the substrates such that the targets are simultaneously sputtered to supply source material to a plasma field for forming a thin film layer on a surface of the substrates. The multiple targets can also be positioned such that a facing axis extending perpendicularly from a center of each target converges at a point on the surface of the substrate.

FIELD OF THE INVENTION

The subject matter disclosed herein relates generally to a system andmethods for deposition of thin films on a substrate, and moreparticularly to a high throughput system for co-sputtering from multipletargets to form thin film layers on photovoltaic module substrates.

BACKGROUND OF THE INVENTION

Thin film photovoltaic (PV) modules (also referred to as “solar panels”or “solar modules”) are gaining wide acceptance and interest in theindustry, particularly modules based on cadmium telluride (CdTe) pairedwith cadmium sulfide (CdS) as the photo-reactive components. CdTe is asemiconductor material having characteristics particularly suited forconversion of solar energy (sunlight) to electricity. For example, CdTehas an energy bandgap of 1.45 eV, which enables it to convert moreenergy from the solar spectrum as compared to lower bandgap (1.1 eV)semiconductor materials historically used in solar cell applications.Also, CdTe converts energy more efficiently in lower or diffuse lightconditions as compared to the lower bandgap materials and, thus, has alonger effective conversion time over the course of a day or inlow-light (e.g., cloudy) conditions as compared to other conventionalmaterials.

Typically, CdTe PV modules include multiple film layers deposited on aglass substrate before deposition of the CdTe layer. For example, atransparent conductive oxide (TCO) layer is first deposited onto thesurface of the glass substrate, and a resistive transparent buffer (RTB)layer is then applied on the TCO layer. The RTB layer may be a zinc-tinoxide (ZTO) layer and may be referred to as a “ZTO layer.” A cadmiumsulfide (CdS) layer is applied on the RTB layer. These various layersmay be applied in a conventional sputtering deposition process thatinvolves ejecting material from a target (i.e., the material source),and depositing the ejected material onto the substrate to form the film.

Solar energy systems using CdTe PV modules are generally recognized asthe most cost efficient of the commercially available systems in termsof cost per watt of power generated. However, the advantages of CdTe notwithstanding, sustainable commercial exploitation and acceptance ofsolar power as a supplemental or primary source of industrial orresidential power depends on the ability to produce efficient PV moduleson a large scale and in a cost effective manner. The capital costsassociated with production of PV modules, particularly the machinery andtime needed for deposition of the multiple thin film layers discussedabove, is a primary commercial consideration.

Accordingly, there exists an ongoing need in the industry for animproved system for economically feasible and efficient large scaleproduction of PV modules, particularly CdTe based modules.

BRIEF DESCRIPTION OF THE INVENTION

Aspects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

Embodiments in accordance with aspects of the invention include a systemfor deposition of a thin film layer on photovoltaic (PV) modulesubstrates. The system can include a sputtering chamber configured toreceive the substrates, at least two targets positioned within thesputtering chamber, and an independent power source connected to eachtarget. Each target can be positioned within the sputtering chamber toface the substrates such that the targets are simultaneously sputteredto supply source material to a plasma field for forming a thin filmlayer on a surface of the substrates. The multiple targets can also bepositioned such that a facing axis extending perpendicularly from acenter of each target converges at a point on the surface of thesubstrate.

Methods are also generally provided for deposition of multiple thin filmlayers on a photovoltaic (PV) module substrate. In one embodiment, themethod can include conveying the substrates on carriers through asputtering chamber and sputtering at least two targets as the substratesmove through the sputtering chamber to form a thin film layer thereon.The at least two targets can be positioned within the sputtering chamberto face the substrates such that the targets are simultaneouslysputtered to supply source material to a plasma field for forming a thinfilm layer on a surface of the substrates. Each of the targets can bepositioned such that a facing axis extending perpendicularly from acenter of each target converges at a point on the surface of thesubstrate.

Methods are also generally provided of manufacturing a cadmium telluridethin film photovoltaic device. The method can include sputtering atransparent conductive oxide layer on a substrate from a first target(e.g., including cadmium oxide) and a second target (e.g., including tinoxide) simultaneously. The first target and second target can bepositioned such that a facing axis extending perpendicularly from acenter of each target converges at a point on the surface of thesubstrate. The facing axis can form an angle with a y-axis that isperpendicular to the surface of the substrate that is about 30° to about60°. A resistive transparent buffer layer can be deposited on thetransparent oxide layer. A cadmium sulfide layer can be deposited on theresistive transparent buffer layer. A cadmium telluride layer can bedeposited on the cadmium sulfide layer.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims. The accompanying drawings, which areincorporated in and constitute a part of this specification, illustrateembodiments of the invention and, together with the description, serveto explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWING

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendedfigures, in which:

FIG. 1 is a cross-sectional view of a CdTe photovoltaic module;

FIG. 2 shows a top plan view of an exemplary system in accordance withaspects of the invention;

FIG. 3 shows a top plan view of an alternative system in accordance withaspects of the invention;

FIG. 4 is a perspective view of an embodiment of a substrate carrierconfiguration;

FIG. 5 is a perspective view of an alternative embodiment of a substratecarrier configuration;

FIG. 6 is diagrammatic view of an embodiment of a sputtering chamber fordeposition of a thin film on a substrate;

FIG. 7 is a diagrammatic view of an alternative embodiment of asputtering chamber; and,

FIG. 8 shows a top view of an exemplary embodiment of co-sputtering fromtwo targets.

Repeat use of reference characters in the present specification anddrawings is intended to represent the same or analogous features orelements.

DETAILED DESCRIPTION OF THE INVENTION

Reference now will be made in detail to embodiments of the invention,one or more examples of which are illustrated in the drawings. Eachexample is provided by way of explanation of the invention, notlimitation of the invention. In fact, it will be apparent to thoseskilled in the art that various modifications and variations can be madein the present invention without departing from the scope or spirit ofthe invention. For instance, features illustrated or described as partof one embodiment can be used with another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncovers such modifications and variations as come within the scope of theappended claims and their equivalents.

In the present disclosure, when a layer is described as “on” or “over”another layer or substrate, it is to be understood that the layers caneither be directly contacting each other or have another layer orfeature between the layers. Thus, these terms are simply describing therelative position of the layers to each other and do not necessarilymean “on top of”since the relative position above or below depends uponthe orientation of the device to the viewer. Additionally, although theinvention is not limited to any particular film thickness, the term“thin” describing any film layers of the photovoltaic device generallyrefers to the film layer having a thickness less than about 10micrometers (“microns” or “μm”).

It is to be understood that the ranges and limits mentioned hereininclude all ranges located within the prescribed limits (i.e.,subranges). For instance, a range from about 100 to about 200 alsoincludes ranges from 110 to 150, 170 to 190, 153 to 162, and 145.3 to149.6. Further, a limit of up to about 7 also includes a limit of up toabout 5, up to 3, and up to about 4.5, as well as ranges within thelimit, such as from about 1 to about 5, and from about 3.2 to about 6.5.

Methods and systems are generally disclosed for co-sputtering thin filmlayers on substrates from multiple targets. Co-sputtering from multiple(i.e., 2 or more) targets can allow more precise control of theelemental species in the sputtering chamber, particularly in thesputtering plasma. As such, the stoichiometry of the deposited thin filmlayer can be more precisely controlled. For example, when forming atransparent conductive oxide (TCO) layer and/or a resistive transparentbuffer (RTB) layer in the production of a photovoltaic device, thestoichiometric chemistry can be more precisely controlled, especiallyduring a high rate sputtering process in a large-scale manufacturingsetting.

Co-sputtering can be achieved from two or more targets formed fromceramic materials, metallic materials, and/or alloyed materials. Theexact composition of the targets can be selected depending on thedesired composition of the thin film layer to be formed. In mostembodiments, each target can have a different composition. As such, eachtarget can supply different elemental species to the sputtering plasma,and ultimately to the deposited thin film layer. Additionally, eachtarget can be substantially the same size and shape as the othertargets, to help ensure that

Additionally, the number of targets can be also selected depending onthe desired composition of the thin film layer to be formed. In oneembodiment, co-sputtering can be achieved using two targets. In otherembodiments, three or more targets can be used. No matter the number oftargets utilized, each target can be positioned to face toward thesubstrate to ensure substantially uniform supply of materials from eachtarget to the substrate in particular embodiments. In particularembodiments, each target can be positioned to face toward the center ofthe substrate. The angles of each target, with respect to a normal axisperpendicular to the face of the substrate, can be adjusted to controlthe mixing of the elemental species supplied from each target to thesubstrate. The spacing between each target can also be adjusted tocontrol the mixing of the elemental species supplied from each target tothe substrate.

FIG. 8 shows an exemplary configuration for co-sputtering from twotargets 201, 202 onto a substrate 10 looking down into the sputteringchamber. The two targets 201, 202 shown in FIG. 8 are positioned to facethe center of the substrate 10, such that the center of each target issubstantially tangent (i.e., perpendicular) to a center point 205 on thesurface of the substrate 12, as represented by the junction of thefacing axis 211 and 212 extending perpendicularly from the center oftargets 201, 202, respectively, meeting at the surface of the substrate12.

The facing axis 211, 212 extending perpendicularly from the center oftargets 201, 202, respectively, form angles θ₁, θ₂ with the y-axis 204that is perpendicular to the surface of the substrate 12 defining thex-axis 203. The angles θ₁, θ₂ can be adjusted to control the elementalspecies supplied from targets 201, 202, respectively, during sputtering.

In one particular embodiment, the angle θ₁ can be substantially equal toangle θ₂, to ensure substantially uniform contribution of elementalspecies from each of the targets 201, 202 to the plasma field 174. Forinstance, each of the angles θ₁, θ₂ can be about 30° to about 60°, suchas about 40° to about 50°. In one particular embodiment, both the anglesθ₁, θ₂ can be about 45°.

In an alternative embodiment, the angles θ₁, θ₂ can be different suchthat one target supplies a disproportionate amount of elemental speciesto the surface of the substrate 12. Adjusting the angles θ₁, θ₂ of eachtarget can help control the rate of sputtering from a particular target,to help control the as-deposited stoichiometry of the layer.Additionally, a different amount of power can be supplied to the firsttarget 201 than the second target 202 in order to individually controlthe sputtering rate from each target.

In one particular embodiment, multiple targets can be utilized todeposit a TCO layer including a combination of cadmium and tin (e.g.,cadmium stannate) onto a substrate. For example, the TCO layer includingcadmium stannate can be deposited from two ceramic targets: (1) acadmium oxide target and (2) a tin oxide target. These targets can besubstantially less expensive than a single target of cadmium stannate,which can help decrease the manufacturing costs of the TCO layer.Alternatively, the TCO layer including cadmium stannate can be depositedfrom two metal targets (e.g., a cadmium target and a tin target)sputtered in a sputtering atmosphere that includes oxygen.

In one particular embodiment, the substrate 12 can be continuouslyconveyed past the targets 201, 202 at a substantially uniform speed.

As mentioned, the present system and method have particular usefulnessfor deposition of multiple thin film layers in the manufacture of PVmodules, especially CdTe modules. FIG. 1 represents an exemplary CdTemodule 10 that can be made at least in part according to system andmethod embodiment described herein. The module 10 includes a top sheetof glass as the substrate 12, which may be a high-transmission glass(e.g., high transmission borosilicate glass), low-iron float glass, orother highly transparent glass material. The glass is generally thickenough to provide support for the subsequent film layers (e.g., fromabout 0.5 mm to about 10 mm thick), and is substantially flat to providea good surface for forming the subsequent film layers.

A transparent conductive oxide (TCO) layer 14 is shown on the substrate12 of the module 10 in FIG. 1. The TCO layer 14 allows light to passthrough with minimal absorption while also allowing electric currentproduced by the module 10 to travel sideways to opaque metal conductors(not shown). The TCO layer 14 can have a thickness between about 0.1 μmand about 1 μm, for example from about 0.1 μm to about 0.5 μm, such asfrom about 0.25 μm to about 0.35 μm.

A resistive transparent buffer (RTB) layer 16 is shown on the TCO layer14. This layer 16 is generally more resistive than the TCO layer 14 andcan help protect the module 10 from chemical interactions between theTCO layer 14 and the subsequent layers during processing of the module10. In certain embodiments, the RTB layer 16 can have a thicknessbetween about 0.075 μm and about 1 μm, for example from about 0.1 μm toabout 0.5 μm. In particular embodiments, the RTB layer 16 can have athickness between about 0.08 μm and about 0.2 μm, for example from about0.1 μm to about 0.15 μm. In particular embodiments, the RTB layer 16 caninclude, for instance, a combination of zinc oxide (ZnO) and tin oxide(SnO₂), and is referred to as a zinc-tin oxide (“ZTO”) layer 16.

The ZTO layer 16 can be formed by sputtering, chemical vapor deposition,spraying pryolysis, or any other suitable deposition method. Inparticular embodiments, the ZTO layer 16 is formed by sputtering (e.g.DC sputtering or RF sputtering) on the TCO layer 14. For example, thelayer 16 can be deposited using a DC sputtering method by applying a DCcurrent to a metallic source material (e.g., elemental zinc, elementaltin, or a mixture thereof) and sputtering the metallic source materialonto the TCO layer 14 in the presence of an oxidizing atmosphere (e.g.,O₂ gas).

The CdS layer 18 is shown on ZTO layer 16 of the module 10 of FIG. 1.The CdS layer 18 is a n-type layer that generally includes cadmiumsulfide (CdS) but may also include other materials, such as zincsulfide, cadmium zinc sulfide, etc., and mixtures thereof, as well asdopants and other impurities. The CdS layer 18 may include oxygen up toabout 25% by atomic percentage, for example from about 5% to about 20%by atomic percentage. The CdS layer 18 can have a wide band gap (e.g.,from about 2.25 eV to about 2.5 eV, such as about 2.4 eV) in order toallow most radiation energy (e.g., solar radiation) to pass. As such,the cadmium sulfide layer 18 is considered a transparent layer on thedevice 10.

The CdS layer 18 can be formed by sputtering, chemical vapor deposition,chemical bath deposition, and other suitable deposition methods. In oneparticular embodiment, the CdS layer 18 is formed by sputtering (e.g.,radio frequency (RF) sputtering) onto the RTB layer 16, and can have athickness that is less than about 0.1 μm. This decreased thickness ofless than about 0.1 μm reduces absorption of radiation energy by the CdSlayer 18, effectively increasing the amount of radiation energy reachingthe underlying CdTe layer 20.

The CdTe layer 20 is shown on the cadmium sulfide layer 18 in theexemplary module 10 of FIG. 1. The layer 20 is a p-type layer thatgenerally includes cadmium telluride (CdTe), but may also include othermaterials. As the p-type layer of the module 10, the CdTe layer 20 isthe photovoltaic layer that interacts with the CdS layer 18 (i.e., then-type layer) to produce current from the absorption of radiation energyby absorbing the majority of the radiation energy passing into themodule 10 due to its high absorption coefficient and creatingelectron-hole pairs. The CdTe layer 20 can have a bandgap tailored toabsorb radiation energy (e.g., from about 1.4 eV to about 1.5 eV, suchas about 1.45 eV) to create the maximum number of electron-hole pairswith the highest electrical potential (voltage) upon absorption of theradiation energy. Electrons may travel from the p-type side (i.e., theCdTe layer 20) across the junction to the n-type side (i.e., the CdSlayer 18) and, conversely, holes may pass from the n-type side to thep-type side. Thus, the p-n junction formed between the CdTe layer 18 andthe CdTe layer 20 forms a diode in which the charge imbalance leads tothe creation of an electric field spanning the p-n junction.Conventional current is allowed to flow in only one direction andseparates the light induced electron-hole pairs.

The cadmium telluride layer 20 can be formed by any known process, suchas vapor transport deposition, chemical vapor deposition (CVD), spraypyrolysis, electro-deposition, sputtering, close-space sublimation(CSS), etc. In particular embodiments, the CdTe layer 20 can have athickness between about 0.1 μm and about 10 μm, such as from about 1 μmand about 5 μm.

A series of post-forming treatments can be applied to the exposedsurface of the CdTe layer 20. These treatments can tailor thefunctionality of the CdTe layer 20 and prepare its surface forsubsequent adhesion to the back contact layer(s) 22. For example, thecadmium telluride layer 20 can be annealed at elevated temperatures(e.g., from about 350° C. to about 500° C., such as from about 375° C.to about 424° C.) for a sufficient time (e.g., from about 1 to about 10minutes) to create a quality p-type layer of cadmium telluride. Withoutwishing to be bound by theory, it is believed that annealing the cadmiumtelluride layer 20 (and the module 10) converts the normally lightlyp-type doped, or even n-type doped CdTe layer 20 to a more stronglyp-type layer having a relatively low resistivity. Additionally, the CdTelayer 20 can recrystallize and undergo grain growth during annealing.

Additionally, copper can be added to the CdTe layer 20. Along with asuitable etch, the addition of copper to the CdTe layer 20 can form asurface of copper-telluride (Cu_(x)Te, where 1≦x≦2) on the CdTe layer 20in order to obtain a low-resistance electrical contact between thecadmium telluride layer 20 (i.e., the p-type layer) and a back contactlayer(s) 22.

The back contact layer 22 generally serves as the back electricalcontact, in relation to the opposite, TCO layer 14 serving as the frontelectrical contact. The back contact layer 22 can be formed on, and inone embodiment is in direct contact with, the CdTe layer 20. The backcontact layer 22 is suitably made from one or more highly conductivematerials, such as elemental nickel, chromium, copper, tin, aluminum,gold, silver, technetium, titanium, or alloys or mixtures thereof.Additionally, the back contact layer 22 can be a single layer or can bea plurality of layers. In one particular embodiment, the back contactlayer 22 can include graphite, such as a layer of carbon deposited onthe p-layer followed by one or more layers of metal, such as the metalsdescribed above. The back contact layer 22, if made of or comprising oneor more metals, is suitably applied by a technique such as sputtering ormetal evaporation. If it is made from a graphite and polymer blend, orfrom a carbon paste, the blend or paste is applied to the semiconductordevice by any suitable method for spreading the blend or paste, such asscreen printing, spraying or by a “doctor” blade. After the applicationof the graphite blend or carbon paste, the device can be heated toconvert the blend or paste into the conductive back contact layer. Acarbon layer, if used, can be from about 0.1 μm to about 10 μm inthickness, for example from about 1 μm to about 5 μm. A metal layer ofthe back contact, if used for or as part of the back contact layer 22,can be from about 0.1 μm to about 1.5 μm in thickness.

In the embodiment of FIG. 1, an encapsulating glass 24 is shown on theback contact layer 22.

Other components (not shown) can be included in the exemplary module 10,such as bus bars, external wiring, laser etches, etc. The module 10 maybe divided into a plurality of individual cells that are connected inseries in order to achieve a desired voltage, such as through anelectrical wiring connection. Each end of the series connected cells canbe attached to a suitable conductor, such as a wire or bus bar, todirect the photovoltaically generated current to convenient locationsfor connection to a device or other system using the generated electric.A convenient means for achieving the series connected cells is to laserscribe the module 10 to divide the device into a series of cellsconnected by interconnects. Also, electrical wires can be connected topositive and negative terminals of the PV module 10 to provide leadwires to harness electrical current produced by the PV module 10.

FIG. 2 represents an exemplary system 100 in accordance with aspects ofthe invention for deposition of multiple thin film layers on PV modulesubstrates 12 (FIG. 4) that are conveyed through the system 100. As afirst matter, it should be noted that the system 100 is not limited byany particular type of thin film or thin film deposition process, asdescribed in greater detail herein.

The illustrated system 100 includes a first processing side 102 whereinsubstrates loaded onto carriers 122 are conveyed in a first directionindicated by the arrow 103. First processing side 102 includes aplurality of different processing stations 104 that are configured fordeposition of a first thin film layer on the substrates as thesubstrates are conveyed along the first processing side 102. Theprocessing stations 104 may include serially arranged modular units thatare aligned to carry out all of the processing steps necessary fordeposition of the first film layer on the substrates. The carriers 122having one or more substrates loaded thereon are introduced into thefirst processing side 102 at an entry location 106. The carriers 122 maybe manually loaded into a load station 152 or, in an alternativeembodiment, automated machinery may be used for introducing the carriers122 into the load station 152. For example, robots or other automatedmachinery may be used for this process.

The carriers 122 are removed from the first processing side 102 at anopposite exit location 108, which may include an external buffer 144.Again, the carriers 122 may be manually unloaded or received byautomated moving equipment, including robotic machines and the like.

The system 100 includes a second processing side 110 that is operablydisposed relative to the first processing side 102 so as to convey thecarriers 122 (and substrates carried thereby) that exit the firstprocessing side 102 in a second direction indicated by the directionalarrow 111 through the second processing side 110. The second processingside 110 includes a plurality of processing stations 112 that areconfigured and serially arranged for deposition of a second thin filmlayer on the first thin film layer. As with the first processing side102, the processing modules 112 along the second processing side 110 areconfigured for carrying out all of the processing steps necessary fordeposition of the thin film layer as the carriers 122 and substrates areconveyed through the second processing side 110.

A first transfer station 118 is operably disposed between the firstprocessing side 102 and the second processing side 110 to receive thesubstrates from the exit 108 of the first processing side 102 and toautomatically move the substrates to an entry 114 to the secondprocessing side 110. The transfer station 118 may include any manner ofautomated machinery for accomplishing the transfer of the carriers 122.For example, the transfer station 118 may include an automated turntable121 that is configured to receive a carrier 122 from the exit 108 of thefirst processing side 102, rotate counter-clockwise 180°, and tointroduce the carrier 122 at the entry 144 of the second processing side110. The turntable 121 may include any manner of robotic or otherautomated machinery for this purpose. In an alternative embodiment, thetransfer station 118 may include any manner of conveyors that accomplishthe task of receiving and conveying the carriers 122 from the exit 108of the first processing side 102 to the entry 144 of the secondprocessing side 110. It should be readily appreciated that any manner oftransfer and conveying configuration may be utilized for this purpose.

In the illustrated embodiments, the first processing side 102 and secondprocessing side 110 are essentially parallel to each others such thatthe direction of conveyance 103 and 111 of the respective processingsides are essentially parallel and opposite in direction. Thisarrangement may be beneficial from the standpoint of saving space in aproduction facility. However, it should be readily appreciated, that thesecond conveying direction may be disposed at any relative operationalangle with respect to the axis of the first processing station 102(including an in-line or zero angle), and that the invention is notlimited to the configuration illustrated in the figures.

With the overall configuration illustrated in FIG. 2, it should readilybe appreciated that the carriers 122 (with substrates) are continuouslymoved through the first and second processing sides 102, 110 fordeposition of multiple thin film layers thereon. The configuration ofFIG. 2 is an open-ended loop configuration wherein the carriers 122 areexternal to the system and introduced into the first processing side 102at the entry location 106. The carriers are subsequently removed fromthe system 100 at the exit location 116 of the second processing side110. This load and unload process may be done manually or by automatedmachinery, as mentioned above.

Still referring to FIG. 2, the various processing stations 104, 112 maybe defined by vertical processing modules 125, with each of theadjacently aligned modules 125 serving a particular processing function,as described in greater detail below. Each of the modules 125 mayinclude an independently driven and controlled conveyor 126. Thesubstrate carriers 122 rest on the conveyors 126 and are thereby movedin a controlled manner through the respective modules 125. In particularembodiments, the conveyors 126 may be roller-type conveyors, beltconveyors, and the like. The conveyors 126 for each of the respectivemodules 125 may be provided with an independent drive (not illustratedin the figure). In an alternative embodiment, a drive may be configuredfor driving multiple conveyors 126 of different modules 125 through anymanner of gearing arrangement. A single conveyor 126 may be associatedwith multiple modules 125.

The various modules 125 are vertically oriented in that the carriers 122convey the substrates in a vertical orientation through the processingsides 102, 110. Referring to FIG. 4, an exemplary carrier 122 isillustrated as a frame-type of structure made from frame members 124.The frame members 124 define receipt positions for substrates 12 suchthat the substrates 12 are horizontally or vertically received (relativeto their longitudinal axis) within the carrier 122. It should beappreciated that the carrier 122 may be defined by any manner of framestructure or members so as to carry one or more of the substrates 12 ina vertical orientation through the processing sides. In the embodimentof FIG. 4, the carrier 122 is configured for receipt of two substrates12 in a horizontal position. It should be readily appreciated that themultiple substrates 12 could also be disposed such that the longitudinalaxis of the respective substrates is in a vertical position. Anyorientation of the substrates 12 within the carrier 122 is contemplatedwithin the scope and spirit of the invention. The frame members 124 maydefine an open-type of frame wherein the substrates 12 are essentiallyreceived within a “window opening” defined by the carrier 122. In analternative embodiment, the carrier 122 may define a back panel againstwhich the substrates 12 are disposed.

The embodiment of the carrier 122 illustrated in FIG. 5 is configuredfor receipt of four substrates 112, wherein pairs of the substrates 12are in a back-to-back relationship. For example, a pair of thesubstrates 12 is disposed in the upper frame portion of the carrier 112,and a second pair of the substrates 12 is disposed in the lower frameportion of the carrier 112. The configuration of FIG. 5 may be used whenfour or more of the substrates 12 are simultaneously processed in thesystem 100, as described in greater detail below with respect to thedeposition apparatus illustrated in FIG. 7.

Referring again to FIG. 2, the first processing side 108 may beparticularly configured with one or more vertical deposition modules 128that define a vacuum sputtering chamber for deposition of a RTB layer ofzinc-tin oxide (ZTO) on the substrates conveyed therethrough. Likewise,the second processing side 110 may include one or more verticaldeposition modules 128 that define a vacuum sputtering chamberparticularly configured for deposition of a cadmium sulfide (CdS) layeron the RTB layer. In one particular embodiment, the zinc-tin oxide (ZTO)layer can be deposited from multiple targets, as shown in FIG. 8. Forexample, the zinc-tin oxide (ZTO) layer can be deposited from a zinctarget and a tin target in the presence of oxygen.

The first processing side 108 may also be particularly configured withone or more vertical deposition modules 128 that define a vacuumsputtering chamber for deposition of a TCO layer of cadmium stannate ona substrate, followed by subsequent deposition of a RTB layer in thesecond processing side 110. For example, the TCO layer can be depositedfrom a multiple ceramic targets, including cadmium oxide and tin oxide,as discussed above.

Operation of vacuum sputtering chambers is well know to those skilled inthe art and need not be described in detail herein. Basically,sputtering deposition generally involves ejecting material from atarget, which is the material source, and depositing the ejectedmaterial onto the substrate in the form of a thin film layer. DCsputtering generally involves applying a voltage to a metal target(i.e., the cathode) positioned near the substrate within a chamber toform a direct-current discharge. The sputtering chamber can have areactive atmosphere (e.g., an oxygen atmosphere) that forms a plasmafield between the metal target and the substrate. The pressure of thereactive atmosphere can be between about 1 mtorr and about 20 mtorr formagnetron sputtering. When metal atoms are released from the target uponapplication of the voltage, the metal atoms react with the plasma anddeposit onto the surface of the substrate. For example, when theatmosphere contains oxygen, the metal atoms released from the metaltarget form a metallic oxide layer on the substrate. RF sputtering is aprocess that involves exciting a capacitive discharge by applying analternating current (AC) or radio-frequency (RF) signal between thetarget source material and the substrate. The sputtering chamber mayhave an inert atmosphere (e.g., an argon atmosphere) having a pressurebetween about 1 mtorr and about 20 mtorr.

FIG. 6 shows a general schematic cross-sectional view of an exemplaryvertical deposition module 128 configured as an RF or DC sputteringchamber 166. A power source 168 is configured to control and supply DCor RF power to the chamber 166. In the case of a DC chamber 166, thepower source 168 applies a voltage to the cathode 170 to create avoltage potential between the cathode 170 and an anode 172. In theillustrated embodiment, the anode 172 is defined by the chamber wall.The glass substrates 12 are held by the carrier 122 so as to begenerally opposite from the cathode 170 (which is also the target sourcematerial 176). A plasma field 174 is created once the sputteringatmosphere is ignited and is sustained in response to the voltagepotential between the cathode 170 and the chamber wall acting as theanode 172. The voltage potential causes the plasma ions within theplasma field 174 to accelerate towards the cathode 170, causing atomsfrom the cathode 170 to be ejected towards the surface of the substrates12. As such, the cathode 170 is the “target” and is defined by thesource material for formation of the particular type of thin filmdesired on the surface of the substrates 12. For example, the cathode170 can be a metal alloy target, such as elemental tin, elemental zinc,or mixtures of different metal alloys. Oxygen in the chamber 166 reactswith the ejected target atoms to form an oxide layer on the substrates12, such as a ZTO layer.

A cadmium sulfide (CdS) thin film layer may be formed in an RFsputtering chamber 166 (FIG. 6) by applying an alternating-current (AC)or radial-frequency (RF) signal between a ceramic target source materialand the substrates 12 in an essentially inert atmosphere.

Although single power sources are illustrated in FIGS. 6 and 7, it isgenerally understood that multiple power sources may be coupled togetherwith a respective target source for generating the desired sputteringconditions within the chamber 166.

FIG. 6 illustrates a heater element 178 within the chamber 166. Anymanner or configuration of heater elements may be configured within thechamber 166 to maintain a desired deposition temperature and atmospherewithin the chamber.

In the embodiment of FIG. 6, the vertical deposition module 128 isconfigured for deposition of a thin film layer on the side of thesubstrates 12 oriented towards the target source material 176. FIG. 7illustrates an embodiment wherein the chamber 166 includes dualsputtering systems for applying a thin film onto the outwardly facingsurfaces of the back-to-back substrates 12 secured in the carriers 122,such as the carrier 122 configuration illustrated and described abovewith respect to FIG. 5. Thus, with the vertical deposition module 128illustrated in FIG. 7, four substrates are simultaneously processed fordeposition of a particular thin film layer thereon.

Although FIGS. 6 and 7 are shown with a single target 176 due to theside view depicted, it should be readily apparent to one of ordinaryskill in the art that multiple targets can be used as discussed aboveand shown in the exemplary embodiment of FIG. 8.

Referring again to the system 100 in FIG. 2, the individual conveyors126 associated with the adjacently disposed vertical deposition modules128 are controlled so as to convey the carriers 122 and attachedsubstrates through the vacuum sputtering chambers at a controlled,constant linear speed to ensure an even deposition of the thin film ontothe surface of the substrates. On the other hand, the carriers 122 andsubstrates are introduced in a step-wise manner into and out of therespective processing sides 102, 110. In this regard, the system 100includes any configuration of entry and exit modules, associatedconveyors 126, and vacuum lock valves 154 with associated controllers156. In addition, the respective processing sides 102, 110 may includeadditional non-vacuum modules at the respective entry and exit sidesthereof for loading the carriers 112 into and out of the system 100,buffering the carriers 122 relative to the transfer station 118, andcooling-down the substrates and carriers 122 prior to removal of thesubstrates from the system 100.

For example, referring to FIG. 2, the first processing side 102 includesa plurality of adjacently disposed vertical processing modules 125. Afirst one of these modules 125 defines a load station 152 wherein thecarriers 122 are loaded into the system. As mentioned, this may be donemanually or robotically. A respective conveyor within the load station152 module moves the carriers 122 to a vacuum load module 132. Thismodule 132 includes an entry vacuum valve 154, which may be, forexample, a gate-type slit valve or rotary-flapper valve that is actuatedby an associated motor 156. The initial valve 154 is open and a carrier122 is conveyed into the module 132 from the load module 152. The entryvalve 154 is then closed. At this point, a “rough” vacuum pump 162 pumpsfrom atmosphere to an initial “rough” vacuum in the millitorr range. Therough vacuum pump 162 may be, for example, a claw-type mechanical pumpwith a roots-type blower. Upon pumping to a defined crossover pressure,the valve 154 between the load module 132 and an adjacent load buffermodule 134 is opened and the carrier 122 is transferred into the loadbuffer module 134. The valve 154 between the modules 132 and 134 is thenclosed, the load module 132 is vented, and the initial valve 154 isopened for receipt of the next carrier 122 into the module. A “high” or“fine” vacuum pump 164 draws an increased vacuum in the load buffermodule 134, and the module 134 may be backfilled with process gas tomatch the conditions in the downstream processing chambers. The finevacuum pump 164 may be, for example, a combination of cryopumpsconfigured for pumping down the module to about less than or equal to9×10⁻⁵ torr.

A process buffer module 136 is downstream of the load buffer module 134and at the prescribed vacuum pressure and conditions within the loadbuffer module 134, the valve 154 between these two modules is opened andthe carrier 122 is conveyed into the process buffer module 136. Thevalve 154 between the modules 134 and 136 is then closed. The processbuffer module 136 serves to essentially convert the step-wise conveyanceof the carriers 122 into a controlled linear conveyance such that theleading edge of the carrier 122 is within a narrow, defined space ordistance (i.e., about 20 mm) from the trailing edge of the previouscarrier 122 so that the carriers 122 are conveyed through the downstreamdeposition modules 128 at a controlled, constant linear speed withlittle space between the respective carriers 122. It should thus beappreciated that, during normal production operations, the valve 154between the process buffer module 136 and first vertical depositionmodule 128 is opened. Likewise, the valve 154 between the adjacentvertical deposition modules 128 is also opened. The valve 154 at theexit of the second vertical deposition module 128 is also opened. Inthis manner, a continuous flow of the carriers 122 through theadjacently disposed vertical deposition modules 128 at a constantprocessing speed is maintained.

Still referring to FIG. 2, an after-process buffer module 138 isdisposed downstream of the last vertical deposition module 128 and thevalve 154 between these modules is opened during normal processing. Asthe carriers 122 leave the vertical deposition module 128 at acontrolled constant linear speed, they enter the after-process buffermodule 138 and are then processed at a greater speed towards theimmediately downstream exit buffer module 140. Prior to this conveyancestep, the valve 154 between the modules 138 and 140 is closed and themodule 140 is drawn down by the fine vacuum pump 164 and backfilled withprocess gas to match the processing zone conditions. Once theseconditions are met, the valve 154 between the chambers is opened and thecarrier 122 is transferred at a relatively higher speed into the exitbuffer module 140. At a predefined crossover pressure between the module140 and a downstream exit module 142 (which may be achieved within themodule 142 by a rough vacuum pump 162), the respective valve 154 betweenthese modules is opened and the carrier 122 is conveyed into the exitmodule 142. The exit module 142 may then be vented to atmosphere. Atthis point, the valve 154 at the exit of the module 142 is opened andthe carrier 122 is conveyed into an external buffer 144.

From the external buffer 144, the carriers 122 are moved into theturntable 121 or other transfer mechanism configured at the transferstation 118. The carriers are rotated or otherwise moved at the transferstation 118 to a position for entry into an external buffer 144 at theentry point of the second processing side 110.

The process buffer module 136 and the after-process buffer module 138may include one or more respective vacuum pump 165, such as aturbomolecular pump, mounted directly to the back of the modules formaintaining the processing vacuum pressures. Likewise, the verticaldeposition modules 128 may also include any manner of vacuum pumps, suchas turbomolecular pumps 165 mounted directly to the back of the modulesbetween each of the cathode pairs associated with the respectivemodules.

Referring again to the system 100 of FIG. 2, the carriers 122 that aretransferred to the external buffer 144 associated with the secondprocessing side 110 are subsequently conveyed through the variousvertical processing modules 125 in essentially the same manner asdiscussed above with respect to the first processing side 102. Theoperation and sequence of the various valves 154, pumps 162, 164, 165,and respective conveyors 126 is as described above for the purpose ofstepping the carriers 122 in a step-wise manner into the processingmodules wherein the carriers 122 are then conveyed at a constant linearspeed through the vertical deposition modules 128. The verticaldeposition modules 128 in the second processing side 110 are configuredfor deposition of a second thin film layer on the first thin film layer,for example a CdS layer, as described above.

After exiting the exit module 142 of the second processing side 110, thecarriers 122 are moved into one or more cool-down stations 148 whereinthe carriers and attached substrates are allowed to cool to a desiredhandling temperature prior to being removed from the system 100. Theremoval process may be manual or automated, for example with roboticmachinery.

The system 100 in FIGS. 2 and 3 are defined by a plurality ofinterconnected modules, as discussed above, with each of the modulesserving a particular function. The respective conveyors 126 configuredwith the individual modules are also appropriately controlled forvarious functions, as well as the valves 154 and associated actuators156. For control purposes, each of the individual modules may have anassociated controller 158 configured therewith to control the individualfunctions of the respective module. The plurality of controllers 158may, in turn, be in communication with a central system controller 160.The central system controller 160 can monitor and control (via theindependent controllers 158) the functions of any one of the modules soas to achieve an overall desired conveyance rate and processing of thesubstrates carried by the carriers 122 as they move through the system100.

It should be readily appreciated that, although the deposition modules128 are described herein in particular embodiments as sputteringdeposition modules, the invention is not limited to this particulardeposition process. The vertical deposition modules 128 may beconfigured as any other suitable type of processing chamber, such as achemical vapor deposition chamber, thermal evaporation chamber, physicalvapor deposition chamber, and so forth. In the particular embodimentsdescribed herein, the first processing side may be configured fordeposition of a ZTO layer, with the vertical deposition modules 128configured as reactive (using oxygen) DC vacuum sputtering chambers.Each module 128 may be configured with four DC water-cooled magnetrons.As mentioned above, each module 128 may also include one or more vacuumpumps mounted on the back chambers between each cathode pair. Thevertical deposition modules 128 associated with the second processingside 110 may be configured as RF vacuum sputtering chambers, with eachmodule 128 including three RF water-cooled magnetrons for deposition ofa CdS layer from a CdS ceramic target material. These modules 128 alsomay include one or more vacuum pumps mounted between the cathode pairs.

FIG. 3 illustrates an alternative system 100 that is similar to thesystem of FIG. 2, but includes a second transfer station 120 between theexit of the second processing side 110 and the entry of the firstprocessing side 102. This particular system thus defines a continuousloop wherein the carriers 122 are continuously conveyed in a processingloop through the system. The carriers 122 move out of the exit module142 of the second processing station 110 and through the cool-downstations 148. The carriers 122 then move into the second transferstation 120, which may be configured as discussed above with respect tothe first transfer station 118. The carriers are transferred from thelast cool-down station 148 to an unload station 150 aligned with thefirst processing side 102. As the carriers 122 move through the unloadstation 150, the substrates are removed from the carriers. Again, thisprocess may be manual or accomplished via automated robotic machinery.The empty carriers then move into a load station 152 wherein newsubstrates are loaded into the carriers 122. The carriers 122 andassociated substrates are then processed through the first and secondprocessing sides 102, 110, as discussed above with respect to FIG. 2.The system 100 in FIG. 3 is unique in that the process is carried out ina continuous-loop manner wherein the carriers 122 need not be removedfrom the system. The efficiency and through-put of the system may besignificantly increased with the configuration of FIG. 3.

The through-put of the system 100 depicted in FIGS. 2 and 3 may befurther increased by utilization of vertical deposition modules 128 asdepicted in FIG. 7 wherein the modules 128 are essentially a combinationof two separate chambers configured in facing relationship so as todeposit the thin-film layers on the surfaces of back-to-back substratesmounted within the carriers 122, as depicted in the carrierconfiguration of FIG. 5.

In the system 100 embodiments of FIGS. 2 and 3, a processing vacuum isseparately drawn and maintained in the respective processing sides 102,110. The carriers are removed from the vacuum processing modules 125along the first processing side 102, transferred to the secondprocessing side 110, and introduced into the vacuum processing modules125 of the second processing side 110 as discussed above. It should bereadily appreciated that the invention also encompasses systems 100wherein an overall vacuum is maintained between the first processingside 102 and second processing side 110. In such a system, the carriers122 would be buffered and transferred from one processing side to theother within a vacuum chamber.

The present invention also encompasses various process embodiments fordeposition of multiple thin film layers on a photovoltaic (PV) modulesubstrate. The processes may be practiced with the various systemembodiments described above or by any other configuration of suitablesystem components. It should thus be appreciated that the processembodiments according to the invention are not limited to the systemconfiguration described herein.

In a particular embodiment, the process includes conveying thesubstrates on carriers in a first direction through a first processingside and depositing a first thin film layer on the substrates as theymove through the first processing side. The carriers are received at theexit of the first processing side and are moved to the entry of a secondprocessing side. The carriers and attached substrates are then conveyedthrough the second processing side for deposition of a second thin filmlayer on the first thin film layer. The substrates are removed from thecarriers at an unload station downstream of an exit from the secondprocessing side and new substrates are placed onto the carriers at aload station upstream of the entry to the first processing side.

The process may include moving the carriers and attached substrates intoand out of vacuum chambers along the first and second processing sidesin a step-wise manner, for example through a series of vacuum locks, yetconveying the carriers and attached substrates through the vacuumchambers at a continuous linear speed during the deposition process.

In a particular embodiment, the first and second processing sides aregenerally parallel and the carriers are moved in a continuous loopthrough the first and second processing sides, with the unload and loadstations being adjacent within the continuous loop.

In another embodiment, the first and second processing sides aregenerally parallel and the carriers are loaded at an entry to the firstprocessing side and removed at an exit of the second processing side.

In still another process embodiment, the thin film layers are depositedwithin vacuum chambers defined along the first and second processingstations, and the carriers and attached substrates are moved through thesystem without breaking vacuum between the first and second processingsides.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they include structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

1. A system for deposition of a thin film layer on photovoltaic (PV)module substrates, the system comprising: a sputtering chamberconfigured to receive the substrates; at least two targets positionedwithin the sputtering chamber to face the substrates such that thetargets are simultaneously sputtered to supply source material to aplasma field for forming a thin film layer on a surface of thesubstrates, wherein the multiple targets are positioned such that afacing axis extending perpendicularly from a center of each targetconverges at a point on the surface of the substrate; and, anindependent power source connected to each target.
 2. The system as inclaim 1, wherein two targets are included within the sputtering chamber.3. The system as in claim 2, wherein the facing axis extendingperpendicularly from the center of each target form an angle with ay-axis that is perpendicular to the surface of the substrate, whereinthe angle formed by each target is about 30° to about 60°.
 4. The systemas in claim 3, wherein the angle formed by each target is about 45°. 5.The system as in claim 3, wherein the angle formed by each target issubstantially the same.
 6. The system as in claim 3, wherein the angleformed by each target is different.
 7. The system as in claim 1, whereina different amount of power is supplied to the each target in order toindividually control the sputtering rate from each target.
 8. The systemas in claim 1 configured such that the substrates are continuouslycarried through the point defined by the convergence of the facing axisof each target.
 9. A method for deposition of multiple thin film layerson a photovoltaic (PV) module substrate, the method comprising:conveying the substrates on carriers through a sputtering chamber; and,sputtering at least two targets as the substrates move through thesputtering chamber to form a thin film thereon, wherein at least twotargets are positioned within the sputtering chamber to face thesubstrates such that the targets are simultaneously sputtered to supplysource material to a plasma field for forming a thin film layer on asurface of the substrates, and wherein the at least two targets arepositioned such that a facing axis extending perpendicularly from acenter of each target converges at a point on the surface of thesubstrate.
 10. The method as in claim 9, wherein each target comprises asource material that is different than the source material of the othertargets.
 11. The method as in claim 9, wherein two targets arepositioned in the sputtering chamber.
 12. The method as in claim 11,wherein the two targets positioned in the sputtering chamber are a firsttarget comprising cadmium oxide and a second target comprising tinoxide.
 13. The method as in claim 11, wherein the facing axis extendingperpendicularly from the center of each target form an angle with ay-axis that is perpendicular to the surface of the substrate, whereinthe angle formed by each target is about 30° to about 60°.
 14. Themethod as in claim 11, wherein the angle formed by each target is about45°.
 15. The method as in claim 11, wherein the angle formed by eachtarget is substantially the same.
 16. The method as in claim 9, whereineach substrates is continuously carried through the point defined by theconvergence of the facing axis of each target at a substantiallyconstant linear speed.
 17. A method of manufacturing a cadmium telluridethin film photovoltaic device, the method comprising sputtering atransparent conductive oxide layer on a substrate from a first targetand a second target simultaneously, wherein the first target comprisescadmium and the second target comprises tin, and wherein the firsttarget and second target are positioned such that a facing axisextending perpendicularly from a center of each target converges at apoint on the surface of the substrate, the facing axis forming an anglewith a y-axis that is perpendicular to the surface of the substrate thatis about 30° to about 60°; depositing a resistive transparent bufferlayer on the transparent oxide layer; depositing a cadmium sulfide layeron the resistive transparent buffer layer; and, depositing a cadmiumtelluride layer on the cadmium sulfide layer.
 18. The method as in claim17, wherein the angle formed by each target is substantially the same.19. The method as in claim 17, wherein the transparent conductive oxideis sputtered on the substrate in a sputtering chamber where thesubstrate is continuously carried through the point defined by theconvergence of the facing axis of each target at a substantiallyconstant linear speed.
 20. The method as in claim 17, wherein adifferent amount of power is supplied to the first target than thesecond target in order to individually control the sputtering rate fromeach target.